The interior of the macrophage is one of the most hostile environments encountered by a microbe. For many pathogens, virulence is associated with the ability to survive inside macrophages. Salmonella typhimurium is an ideal organism to study intracellular survival. The modulated expression of Salmonella proteins during infection of macrophages suggests that a specific subset of genes and their proteins enable Salmonella to survive within macrophages. The long term objective of this research is to understand how Salmonella typhimurium regulates expression of these genes in response to the intracellular environment of the macrophage, to identify these genes and to determine how they enable Salmonella to survive in this hostile environment. The regulatory network defined by a specific macrophage sensitive mutant of Salmonella typhimurium (MS4347) will be characterized. This will include mapping and sequencing the genes involved in this regulatory network, determining how the genes are regulated and identifying the protein products of these genes. The ultimate goal is understanding how the regulated gene products contribute to the survival of Salmonella in macrophages. These studies will define how a bacterial genetic regulatory system functions within the complex and hostile environment of the macrophage. It will also identify specific genes which are necessary for Salmonella survival in macrophages. This information may then be used in the identification of similar genes from other intracellular bacteria and will provide a greater understanding of how intracellular microbes survive.